1. Field of the Invention
Embodiments of the present invention generally relate photomasks used in the fabrication of semiconductor devices, more specifically, to extreme ultraviolet light (EUV) photomasks and methods of etching thereof.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created on a series of reusable photomasks (also referred to herein as masks) in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. The masks are used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
A mask generally comprises a transparent substrate having an opaque, light-absorbing layer disposed thereon. Conventional masks typically include a glass or quartz substrate having a layer of chromium on one side. The chromium layer is covered with an anti-reflective coating and a photosensitive resist. During a patterning process, the circuit design is written onto the mask, for example, by exposing portions of the resist to an electron beam or ultraviolet light, thereby making the exposed portions soluble in a developing solution. The soluble portion of the resist is then removed, allowing the exposed underlying chromium and anti-reflective layers to be etched (i.e., removed).
With the shrink of critical dimensions (CD), present optical lithography is approaching a technological limit at the 45 nanometer (nm) technology node. Next generation lithography (NGL) is expected to replace the current optical lithography method, for example, in the 32 nm technology node and beyond. There are several NGL candidates such as extreme ultraviolet (EUV) lithography (EUVL), electron projection lithography (EPL), ion projection lithography (IPL), nanoimprint, and X-ray lithography. Of these, EUVL is the most likely successor due to the fact that EUVL has most of the properties of optical lithography, which is a more mature technology as compared with other NGL methods.
However, EUV mask fabrication still has technological challenges to overcome. For example, EUV mask etch process optimization is still in the pioneering stage. The key challenges of EUV mask fabrication include etch CD bias control, etch CD uniformity, cross sectional profiles, etch CD linearity, etch selectivity, and defectivity control. Due to the tight specifications and reduced CD tolerances of EUV masks, CD control becomes more critical. A nearly zero etch CD bias is expected to be required to meet the mean to target (MTT) CD requirement and uniformity control.
The main etch CD bias issues result from erosion of the photoresist—which is a soft mask. Final mask CD properties are the contribution of pattern generation and pattern transfer processes (etch). Some intrinsic CD non-uniformity may exist prior to etch, such as fogging effects in the photoresist due to the e-beam writing process. A thinner resist layer is helpful to control this non-uniformity, but the resist thinness is limited by the thickness of subsequently etched layers due to limited etch selectivity (e.g., during pattern transfer, resist is consumed significantly because of the limited etch rate selectivity of absorber materials to resist). The more resist is consumed, the lower the fidelity of a pattern transfer process.
To overcome the photoresist limitation, use of a hard mask was proposed for the CD control. However, an extra hard mask will make mask fabrication much more complicated. When the hard mask finishes its function, it has to be removed without affecting other layers (e.g., without affecting the absorber layer and buffer/capping layers, and without introducing any defects to the mask). This imposes a high mask selectivity requirement, thereby making EUV mask fabrication even more challenging. High cost and low production yields caused by use of a hard mask are additional concerns.
Thus, there is a need for an improved EUV mask and fabrication method.